Checking Valve Clearances Hot vs. Cold

Hey Gang - I've had some old bike mechanics offer different views on checking the shims on a hot engine versus a cold one.

I always thought that this was best done on a cold engine but one veteran got me thinking with his check 'em when they are hot theory as he says the expansion from the heat is how the engine normally operates.

Anyone care to chime in on this?

Cheers-Colin Firth-Ontario Canada

My first thoughts...
I see the point about the engine being warm. But the clearances are designed to allow the engine to warm and the parts expand. That is why valve clearances are not at zero. Plus the shim is designed to wear some.
I have never heard anyone say to make sure the engine is warm first when installing a shim... the reason for this might be time. It could take a significant amount of time once the engine is shut off to get to the camshaft and shims -- how would you keep the engine warm as you remove parts of the engine to get to the camshaft and move from piston to piston? A torch? Engine work in leather gloves seems unreasonable.

I think all the books say to check cold, I would think they took into account the expansion of the parts whe they wrot the specs.
I check mine cold, plus I dont roast a finger:P

i think it a bit goofy to hop on a bike drive it around until its boiling hot and then pull it apart in time to test clearances with hot parts

From the time it takes to shut off the engine, wheel the bike inside, take off the tank, coils and to remove the valve cover this is around 10 minutes for me. I suspect the engine hasn't lost that much of its operating temperature in this short of a period but I don't have any data to back this suspicion.

A mechanic once told me that it takes a car engine nearly 24 hours to return to surrounding temperatures because of the block and fluids retaining heat. Obviously this is dependent upon a number of variables, most importantly the surrounding air temperature.

BTW, anytime I work on my bike and the motor or exhaust is hot I wear mechanix gloves. I learned this the hard way.

I guess that I should've rephrased my question to - Is there much of a difference is valve clearance between a cold motor and one that is still near operating temperature? In my 4 years of hanging around Kzrider I cannot recall this question being put forward.

I suspect that BSK650 is correct in mentioning that the operating temperature was taken into account when the engine was originally designed.

Cheers-Colin Firth-Ontario Canada

in a couple a months i can let you know when i take my thermodynamics class for metals

The KZ650 Kawasaki Service Manual is very specific in this area. It states: "NOTE: Valve clearance must be checked when engine is cold." I suspect that the folks that designed the engine and wrote the book put the word "MUST" in that note for a good reason. I doubt they were worried about you or me burning our fingers. Ed

I think like pretty much said before, the reason we want .10-.15mm clearance "cold" is just for that reason, the engine is cold. When warm the clearance will decrease supposedly as that's the theory and I've never cared to check into it further. I suppose if you knew how much difference cold vs a warm engine you could compensate and do it warm, but you'd probably need to be between zero and .05mm clearance warm to be in spec would just be a guess.

Probably akin to the same reasons for checking tire air pressure when tires are cold

What is the reason for having clearance between the cam caps and the cam? It's simply to allow for the expansion due to heat. In an ideal situation you would want to have exactly zero clearance when the motor is at operating temp, that way you would obtain the maximum lift and duration of the cam lobe.

If you know the maximum temp that the motor will reach during operation, then you can calculate the required cold clearance. All metals have known thermal expansion rates (you can look them up online if you are interested), so if you determine the length of the valve from seat to tip, and the distance between the valve seat and the cam lobe, you can calculate the increase in valve length, and the expansion of the head, to determine the needed cold clearance. Thankfully Kawasaki has done this for us.

This is actually commonly done in accurate machining operations. For instance in production shops, it's common to leave the grinders in the grinding room running all weekend so the employees don't have to stand around for six hours waiting for the machines to warm up to operating temp to reach a stable temp. Otherwise the clearnces will change during operation and they would have to throw away a bunch of parts for failing to meet the standards for accuracy.

I once calculated the change in length due to heat expansion of the six foot cast iron bed of my lathe, IIRC (it's been awhile) when warming from 0F to 100F the lathe bed would grow a little over .125" (1/8") in length.

The secretary at work gets perturbed with me when she goes to put her lunch (a frozen dinner) in the freezer and finds it stuffed full of parts. I stick parts (bushings, pins, etc) in the freezer to shrink them so I can install them in situations that call for a press fit, otherwise I would never get them to fit in the hole. That's an example of using the thermal expansion (and contraction) of metals to my advantage

Here is a link to a chart of thermal expansion values for some metals.

Thermal expansion of some common metals

isnt he dreamy ?

He is dreamy...nice post and some great insight steell.

Personally, I'm glad we had the engineers at Kawasaki Heavy Industries to cypher all of this out for us years ago. :huh: